In total, 626 individuals of Alburnus mossulensis and 586 individuals of Acanthobrama marmid were collected monthly from Karakaya Dam Lake on the upper Euphrates River. The gonadosomatic index, somatic condition, oocyte size, and fecundity were calculated on a monthly basis. Reading of scales indicated that the maximum age was 4+ years for A. mossulensis and A. marmid. The estimated length-weight relationships were W = 0.206 x FL2.065 for females and W = 0.119 x FL2.138 for males in A. mossulensis, and W = 0.029 x FL2.678 for females and W = 0.030 x FL2.631 for males in A. marmid. Growth in length equations were L-t = 19.6[1 - e(-0.14(t + 1.39))] for females and L-t = 20.1[1 - e(-1.40(t + 1.04))] for males in A. mossulensis and L-t = 17.3[1 - e(-1.37(t + 1.04))] for females and L-t = 16.6[1 - e(-1.29(t + 1.04))] for males in A. marmid. This study investigated oocyte size and fecundity for A. mossulensis and A. marmid in Turkey for the first time. The spawning period was between May and August for A. mossulensis and between May and June for A. marmid. These 2 species usually play a key role in the food web of temperate freshwater systems. In addition to water quality, biological data of these 2 species are very important in terms of future water management of the basin of Karakaya Dam Lake.